In order to perform sharpness processing to a color space represented by luminance and color components, such as a CIELAB color space, a CIELUV color space and a YCbCr color space, conventional image processing devices determine the correction value for the luminance component according to the saturation value of the pixel, and at the same time correct the color component. In this manner, problems such as color turbidity after the correction are solved.
As a conventional technique of an image processing device, Japanese Patent Laid-Open Application No. 2000-175046 (Related Art 1) discloses an image processing method and an image processing device for noise reduction and sharpness enhancement of a digital image.
The image processing device described in Related Art 1 is provided with a sharpness processor, a smoothing processor, an edge/noise-mixed component extractor, an edge detector, a noise region weighting factor operator, a noise component identifier and an output image operator.
The sharpness processor performs sharpness enhancement to the original image data to sharpen the image and the noise included in the image, thereby generating sharpness-enhanced image data. The smoothing processor smoothes the original image data to generate a smoothed image data. The edge/noise-mixed component extractor subtracts the smoothed image data from the sharpness-enhanced image data to generate an edge/noise-mixed object image data including both of the sharpness-enhanced edge of the object image and the sharpness-enhanced noise. The edge detector detects the edge of the original image data to obtain weighting data of the edge region for identifying the edge and noise regions of the object image. The noise region weighting factor operator obtains weighting data of the noise region from the obtained weighting data of the edge region. The noise component identifier multiplies the edge/noise-mixed object image data by the noise region weighting data to obtain noise data for each color in the noise region, calculates color correlation of the noise data for each color, obtain black-and-white noise components and color noise components present in all colors based on the obtained color correlation components and multiply both of the obtained black-and-white noise components and color noise components by their reduction factor, thereby obtaining a black-and-white noise reduction component and a color noise reduction component. The output image operator selectively removes the black-and-white noise reduction component and the color noise reduction component from the sharpness-enhanced image data to reduce the noise, thereby generating a processed image that retains sharpness enhancement in the image edge region.
Japanese Patent Laid-Open Application No. 2000-175047 (Related Art 2) discloses an image processing method and an image processing device for noise reduction and sharpness enhancement of a digital image.
The image processing device described in Related Art 2 is provided with a sharpness processor, a smoothing processor, an edge/granularity-mixed component extractor, an edge detector, a granularity region weighting factor operator, a granularity component identifier and an output image operator.
The sharpness processor performs sharpness enhancement to the original image data to sharpen the image as well as the granularity or the noise included in the image, thereby generating sharpness-enhanced image data. The smoothing processor performs smoothing process to the original image data to generate a smoothed image data. The edge/granularity-mixed component extractor subtracts the smoothed image data from the sharpness-enhanced image data to generate an edge/granularity-mixed object image data that includes both of the edge of the sharpness-enhanced object image and the sharpness-enhanced granularity. The edge detector detects the edge of the original image data to obtain weighting data of the edge region for identifying the object image edge region and the granularity region. The granularity region weighting factor operator obtains weighting data of the granularity region from the obtained weighting data of the edge region. The granularity component identifier multiplies the edge/granularity-mixed object image data by the granularity region weighting data to obtain granularity data for each color in the granularity region, calculates, based on the granularity data of each color, a local granularity factor indicating the spatial size and the magnitude of a change in density of the granularity, identify and separate black-and-white granularity components and pigment granularity components, and multiply both of the obtained black-and-white granularity component and the pigment granularity component by their reduction factor, thereby obtaining a black-and-white granularity reduction component and a pigment granularity reduction component. The output image operator selectively removes the black-and-white granularity reduction component and the pigment granularity reduction component from the sharpness-enhanced image data to reduce granularity, thereby generating a processed image that retains sharpness enhancement in the image edge region.
Japanese Patent Laid-Open Application No. 2000-278542 (Related Art 3) discloses an image processing device which is capable of natural edge enhancement between colors that have little difference in lightness and densities.
The image processing device described in Related Art 3 processes and outputs color image information input from a certain image input device. The image processing device is provided with a color difference information extracting means, a color difference edge detecting means, a color difference enhancement level calculating means and a color difference enhancement correcting means. The color difference information extracting means extracts color difference information from the color image information. The color difference edge detecting means detects a color difference edge from the color difference information extracted by the color difference information extracting means, thereby calculating the color difference edge value at the color difference edge. The color difference enhancement level calculating means calculates a color difference enhancement level based on the color difference edge value calculated by the color difference edge detecting means. The color difference enhancement correcting means performs enhancement correction to the color difference information based on the color difference enhancement level calculated by the color difference enhancement level calculating means.
Japanese Patent Laid-Open Application No. 9-284560 (Related Art 4) discloses a sharpness processing device which is capable of providing an excellently-sharpened printed matter at a low cost.
The sharpness processing device described in Related Art 4 performs sharpness processing to an input image signal. The device is provided with a storing means, a selecting means, a processing means and an outputting means. The storing means stores a plurality of values corresponding to a plurality of image display conditions as parameters for sharpness processing. The selecting means selects a particular value from the plurality of parameters according to a particular image display condition selected among the plurality of image display conditions. The processing means performs sharpness processing to the input image signal in accordance with the particular value. The outputting means outputs an output image signal resulting from the sharpness processing to an image displaying means that is appropriate for the particular image display condition. The sharpness processor described in Related Art 4 is characterized in that the plurality of parameters give substantially equal sharpness presentations for the plurality of image display conditions.
Japanese Patent Laid-Open Application No. 10-42152 (Related Art 5) discloses an image processing device and method in which a circuit configuration required for edge enhancement correction is simplified.
The image processor described in Related Art 5, which performs sharpness correction to an input image signal, is provided with an extracting means, an edge value extracting means, a saturation extracting means, a dividing means and an edge enhancement means. The extracting means extracts a lightness signal and a chromaticity signal from of the input image signal. The edge value extracting means extracts an edge value based on the lightness signal. The saturation extracting means extracts a saturation component based on the chromaticity signal. The dividing means divides the enhancement level of the edge portion, which is based on the edge value extracted by the edge value extracting means, for a lightness signal and a chromaticity signal according to the saturation component extracted by the saturation extracting means. The edge enhancement means corrects the lightness signal and the chromaticity signal by the enhancement levels of the edge portion divided by the dividing means.
From Japanese Patent No. 2906975 (Related Art 6), a color image processing method and device which can reduce an image noise caused by edge enhancement processing and which can perform visually natural edge enhancement are known.
The image processing device described in Related Art 6 is provided with an edge detecting means for obtaining an edge value signal from a luminance signal of a luminance/chromaticity-separated signal, an edge enhancement means for enhancing an edge portion in an image represented by the luminance signal, a smoothing means for smoothing the luminance signal, a saturation detecting means for detecting a saturation signal of the image based on the chromaticity signal of the luminance/chromaticity-separated signal, a means for converting luminance by mixing the output from the edge enhancement means and the output from the smoothing means based on the edge value signal detected by the edge detecting means, and means for converting chromaticity of the chromaticity signal corresponding to compression or expansion of saturation using the output from the edge detecting means and the output from the saturation detecting means.
In order to correct color components, conventional image processing devices perform sharpness processing to the color components by extracting and using an edge value based on the changing point (edge) of the lightness (characteristic) of the image through edge detection of the luminance component. In this case, sharpness processing cannot be performed in a satisfactory manner when values of the luminance components at an edge portion of a letter or a line are generally equal but values of the color components are entirely different. For example, when a yellow letter or a yellow line is on a white background, conventional image processing devices cannot sufficiently cope with the difference of any of luminance, saturation and hue components of the pixels in the color image since the luminance of white and yellow colors are generally equal.